Every gas prep experiment ends the same way: you’ve made a gas, and now you actually have to catch it. Get the method wrong and you lose most of your product — or worse, you inhale something you shouldn’t have. Collecting gas properly comes down to answering two questions about the gas you’re working with, and once those two questions are answered, the right method almost picks itself.
Collecting gas means capturing a gas produced in a chemical reaction so it can be stored, measured, or tested. The method depends on two properties: whether the gas is denser or less dense than air, and whether it dissolves in water. Gases denser than air are collected by downward delivery; less dense gases by upward delivery; and gases that don’t dissolve in water can be collected over water.
What Decides How You Collect a Gas?
Before touching any apparatus, ask two questions.
Is the gas denser or less dense than air? Air has a density of roughly 1.2 g per litre. Carbon dioxide and chlorine are heavier than this, so they sink and pool in a container held upright. Hydrogen and ammonia are lighter, so they rise and escape unless you trap them from below.
Is the gas soluble in water? If a gas dissolves readily — ammonia and hydrogen chloride are the classic examples — you can’t collect it over water, because it would simply disappear into the liquid instead of displacing it.
Once you know the answer to both, the collection method is a formality. This is also exactly the logic examiners test: they’ll give you a gas’s density and solubility and expect you to pick the method, not just recall a memorised list.
Downward Delivery — For Gases Denser Than Air
Downward delivery, sometimes called “collection by downward displacement of air,” is used for gases heavier than air. The delivery tube runs to the bottom of an upright gas jar or beaker, and the gas sinks in and pushes the lighter air out the top as it fills.
Carbon dioxide, chlorine, sulfur dioxide, and hydrogen chloride are all collected this way in standard school practicals. It’s a simple setup with no water trough required, which is part of why it’s the go-to method whenever the gas is both dense and reactive with water.
Quick Takeaway: If a gas is heavier than air, point the delivery tube down into an open jar and let gravity do the work.
Upward Delivery — For Gases Less Dense Than Air
Upward delivery is the mirror image. Because a light gas rises, the collecting vessel is held upside down over the delivery tube, and the gas fills it from the closed top downward, pushing air out at the open bottom.
Hydrogen is the textbook example — it’s roughly fourteen times less dense than air. Ammonia is collected the same way for the same reason. This method has a well-known real-world echo: the Hindenburg airship was kept aloft by roughly 140,000 cubic metres of hydrogen precisely because the gas is so much lighter than the surrounding atmosphere, and its 1937 disaster is still used in textbooks to illustrate both the buoyancy and the flammability of hydrogen gas.
Collecting Gas Over Water
This method — also called water displacement — works for any gas that’s insoluble or only very slightly soluble in water, regardless of whether it’s denser or lighter than air. A test tube or gas jar is filled completely with water, inverted, and placed under the surface of a trough. The delivery tube feeds gas bubbles up into the container, and as the gas collects at the top, it pushes the water out the open bottom.
Oxygen and hydrogen are both routinely collected this way, since neither dissolves in water to any meaningful degree. It’s the neatest of the three methods because you get a visibly measurable, air-free sample with almost no contamination — but it’s a non-starter the moment the gas is soluble, which rules it out immediately for ammonia, hydrogen chloride, and sulfur dioxide.
The Gas Syringe Method
A gas syringe sidesteps the density-and-solubility question entirely. Because it’s a sealed, calibrated cylinder connected directly to the reaction vessel by a delivery tube, it works for essentially any gas, soluble or not, dense or light.
Its real advantage isn’t just collection — it’s measurement. As gas enters the syringe, the plunger is pushed back, and you can read the volume directly off the scale at set time intervals. Plotting that volume against time gives a rate-of-reaction graph: a steep early section shows gas being produced quickly (more reactant particles, more frequent successful collisions), and the curve flattening out shows the reaction slowing as a reactant is used up. That makes the gas syringe the standard tool whenever an experiment is actually about rate, not just about getting a gas sample.
An upturned measuring cylinder over water can be used for the same rate-tracking purpose, and a mass-loss method (weighing the reaction flask as gas escapes) is a third alternative — useful in particular when the gas produced is one you don’t want to trap and store.
Drying a Gas After You’ve Collected It
This is the step most revision guides skip, and it’s exactly why it’s worth knowing well. A gas collected by downward or upward delivery in a lab setup can pick up moisture from the air in the apparatus, and for some experiments that trace water vapour has to be removed before the gas is used or tested.
The standard drying agent is concentrated sulfuric acid, bubbled through in a Drechsel bottle, or a solid desiccant like calcium chloride or silica gel packed into a drying tube. Here’s the catch that catches students out in exams: concentrated sulfuric acid cannot be used to dry ammonia. Ammonia is a base, sulfuric acid is an acid, and the two react with each other rather than simply drying the gas — so ammonia is dried with a different desiccant, such as calcium oxide, instead.
Testing the Gas You’ve Collected
Once a gas is safely in the jar or tube, you confirm what it is with a short, specific test:
- Oxygen: insert a glowing splint — it relights instantly.
- Hydrogen: hold a lit splint at the mouth of the tube — you’ll hear a “squeaky pop.”
- Carbon dioxide: bubble the gas through limewater — it turns milky or cloudy.
- Ammonia: hold damp red litmus paper in the gas — it turns blue, and the smell is unmistakably pungent.
- Chlorine: hold damp blue litmus paper in the gas — it turns red, then bleaches to white.
These tests aren’t just a memory exercise. Each one works because of a genuine chemical property — oxygen supports combustion, hydrogen is flammable, carbon dioxide reacts with the calcium hydroxide in limewater to form insoluble calcium carbonate, ammonia is alkaline in solution, and chlorine’s bleaching action comes from the chloric(I) acid it forms in water.
Comparison Table — Which Method for Which Gas?
| Gas | Denser/Lighter than Air | Soluble in Water? | Collection Method |
|---|---|---|---|
| Hydrogen | Lighter | No | Upward delivery or over water |
| Oxygen | Slightly denser | No | Over water (or downward delivery) |
| Carbon dioxide | Denser | Slightly soluble | Downward delivery |
| Ammonia | Lighter | Very soluble | Upward delivery only |
| Chlorine | Denser | Soluble | Downward delivery only |
| Hydrogen chloride | Denser | Very soluble | Downward delivery only |
| Sulfur dioxide | Denser | Soluble | Downward delivery |
Common Mistakes Students Make (and How to Avoid Them)
The most frequent error is picking a method based on density alone and forgetting solubility — chlorine is dense enough that a student might reach for downward delivery correctly, but then try to “double check” by collecting it over water, which fails because chlorine dissolves and reacts with the water.
The second common mistake is assuming a gas syringe is always the “safe default.” It’s versatile, but it doesn’t suit every practical — corrosive or toxic gases like chlorine still need to be handled inside a fume cupboard regardless of collection method, following the relevant CLEAPSS Hazcard for that gas, not just contained in a syringe.
The third is skipping the drying step conceptually — students often can describe a method perfectly but forget why ammonia needs a different desiccant, which is precisely the kind of applied-reasoning question that separates a strong exam answer from an average one.
Quick Takeaway
Two questions settle almost every gas collection question you’ll ever face: is it denser or lighter than air, and does it dissolve in water? Answer both, and you’ll know instantly whether you’re reaching for downward delivery, upward delivery, water displacement, or a gas syringe — and you’ll know which desiccant is safe to dry it with afterward.
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FAQ Section
Q1: What are the three main methods of collecting gas? The three core methods are downward delivery (for gases denser than air), upward delivery (for gases less dense than air), and collection over water (for gases that don’t dissolve in water). A gas syringe is a fourth, more universal option used mainly to measure volume.
Q2: How do you know which method to use to collect a gas? Check two properties: whether the gas is denser or lighter than air, and whether it’s soluble in water. Denser, insoluble gases go by downward delivery; lighter, insoluble gases go by upward delivery; and any insoluble gas can also be collected over water.
Q3: Can all gases be collected over water? No. Only gases that are insoluble or only slightly soluble in water can be collected this way. Soluble gases like ammonia and hydrogen chloride would simply dissolve into the water instead of displacing it.
Q4: What is the test for carbon dioxide gas? Bubble the gas through limewater. If carbon dioxide is present, the limewater turns milky or cloudy because insoluble calcium carbonate forms.
Q5: Why is chlorine collected by downward delivery instead of over water? Chlorine is denser than air, which suits downward delivery, but it also dissolves in and reacts with water to form a weakly acidic solution — so collecting it over water would both lose product and contaminate the sample.
Q6: What is a gas syringe used for? A gas syringe collects gas in a sealed, calibrated cylinder and gives a direct volume reading, making it the standard tool for tracking how much gas is produced over time — for example, when measuring the rate of a reaction.
Q7: How do you dry a gas safely? Most gases can be dried by passing them through concentrated sulfuric acid or a solid desiccant such as calcium chloride. Ammonia is the exception — as a base, it reacts with sulfuric acid, so it must be dried with a different agent, such as calcium oxide.
Q8: Why is hydrogen collected by upward delivery? Hydrogen is around fourteen times less dense than air, so it rises rather than sinking. Collecting it in an inverted container held over the delivery tube lets it fill from the top down as air is pushed out below.